The present invention relates to a method for determining the distance between a relief well and a blowout well. The invention is also useful in determining which formation is producing in a blowout well that contains multiple producing formations. During the drilling of wells for the production of hydrocarbons, at times conditions arise which cause either an imbalance of mud weight or loss of drilling mud or borehole fluid in the borehole and results in the well running wild or blowing out. The pressure of the borehole fluid column in the borehole can become underbalanced with respect to a formation by using too light a borehole fluid. Loss circulation occurs when the mud weight is too high and hydraulically fractures a formation which allows the mud to flow into the formation, making it impossible to keep the wellbore full of borehole fluid. Of course, it is the normal drilling practice to maintain the weight of the borehole fluid so that the pressure produced by the column of borehole fluid in the well will exceed the pressure of any formation penetrated by the well.
Blowouts can also occur during remedial workovers of producing wells. During a workover, it is conventional to fill the borehole with drilling mud or borehole fluid having a weight sufficient to hold back formation pressure. Remedial actions can be carried out to improve the well performance or replace tubing strings or other production equipment in the well. During these remedial operations, at times the borehole fluid in the well is lost or for other reasons it is not sufficient to contain the formation pressure. The formation fluids then displace the borehole fluid and the well runs wild.
In many blowouts, the wellhead equipment is destroyed and/or the well catches fire. Since the wellhead equipment is destroyed in such cases, it is impossible to contain the blowout well by closing valves or blowout preventors at the surface and other methods must be utilized. The most common practice in controlling such a blowout well is to drill a relief well which intercepts the formation which is feeding the blowout well. After the interception is effected, heavy fluids and/or cement is pumped down the relief well to kill the formation. While the method is well understood, the ability to accurately guide a relief well so that it intercepts the formation sufficiently close to the blowout well to permit killing the formation is at times difficult. Also, in the case of extremely deep wells, for example 20,000 feet, it has been the practice to attempt an interception of the blowout well at an intermediate depth and pump the heavy cement down the blowout well to stem flow from the formation. Regardless of the type of relief well being drilled, one must know the position of the relief well and the distance between the relief well and the blowout well in order to effect the closing between wells or interception, as the case may be.
In the past various magnetic and electrical methods have been utilized to locate a blowout well with respect to a relief well. For example, U.S. Pat. No. 3,725,777 describes a method using magnetic measurements to locate the blowout well when the blowout has been cased with casing having a remnant magnetism. This method has been further extended now to locate any cased well from an open relief well and has proven satisfactory in many relief well applications. The method does require casing or other tubulars to be present and information regarding the magnetic characteristics of the formation and the casing or other tubulars in the blowout well. Another method for determining the distance between a relief well and a blowout well is described in U.S. Pat. No. 3,748,574. This patent utilizes resistivity measurements made in the open well using an electrical logging device having extremely long electrode spacings. While the method has been used satisfactorily, it does require some knowledge of the resistivities of the formations between the relief and the blowout wells. This information at times can be obtained from conventional resistivity logs run in the relief well.